After a little digging I discovered the reason, the reply tag was followed by -- which appears to run afoul of the rules of the parser that drives inbox notification. The explanation lies in the answer to this question, Why was I not notified?

Since @replies are only valid to the users who have left comments previously, it should be possible to be more aggressive about matching usernames even in the case of usernames with punctuation. If a tooltip suggestion for the username is accepted, there's no need to even guess, the username is known precisely.

If nothing else is possible, please consider appending a zero width space to the username.